The need for human wastewater collection and treatment has been recognized for centuries. Initially, this need was driven by the desire to reduce disease caused by humans living in close proximity to their waste, but more recently wastewater treatment methods have evolved with a desire to reduce or eliminate water pollution and achieve desired levels of environmental water quality. In the United States in the 1800s, the first major evolution of wastewater disposal began when pit privies and open drainage ditches were replaced by buried sewers that transmitted wastes and stormwater to other locations where it would have less effect on the community. The sewered population rapidly increased from about 1 million in 1860 to about 25 million in 1947 reflecting public awareness of the link between human disease and waste disposal practices.
Once large quantities of wastewater began being collected by sewers, it became possible to develop centralized treatment processes to reduce or eliminate the harmful effects of sewerage on human health and the environment. The first treatment methods were basically anaerobic processes where collected sewage was allowed to digest and stabilize essentially undisturbed. However, near the end of the 1800s several researchers, with the idea that aerobic treatment would avoid malodorous anaerobic conditions and undesirable results, began to explore blowing air into sewage tanks. Over the course of the next three decades, experiments in aerobic treatment of sewerage lead to the conclusion that aerating wastewater in the presence of a suspended biomass (achieved through solids recycle) was a very effective method of treating wastewater to degrade the biological constituents in the wastewater. In 1914 this process was coined the “activated sludge process” and has since become the standard method for secondary wastewater treatment.
The activated sludge process is a biochemical type of reaction. It involves the mass transfer of oxygen from an oxygen containing gas into the wastewater and then the mixing and use of that dissolved oxygen to support the growth of aerobic microorganisms suspended in the wastewater. These microorganisms, known as the biomass, oxidize the organic materials in the wastewater in different ways to eliminate the biochemical oxygen demand of the wastewater. FIG. 1 depicts a simple schematic diagram of a typical, continuous flow, modern activated sludge process for “secondary” wastewater treatment. Effluent from “primary” treatment, which typically involves just grinding and settling in a primary clarifier, is the influent 15 to secondary treatment. The influent typically also comprises recycle biological solids 16 (activated sludge), and both the primary effluent and recycle activated sludge are mixed and aerated in a covered or uncovered biochemical reactor 17. Note that the primary effluent 15 and the recycle activated sludge 16 are usually fed separately into the aeration basis and are not necessarily combined prior to entering the basin as suggested by the schematic. Oxygen necessary for the process is provided by air or oxygen enriched gas 20 and aeration is usually hastened by use of gas-liquid contacting devices such as diffusers, surface aerators, and sparging mixing impellers (not shown). Usually the process operates in a continuous-flow mode, but it can also be operated as a semi-batch process. The contents of the aeration reactor basin are referred to as mixed liquor and contain microorganisms (living and dead), soluble contaminants, and inert, biodegradable, and non-biodegradable suspended and colloidal matter. The particulate solid fraction of the mixed liquor is termed mixed liquor suspended solids (MLSS).
After a sufficient residence time for the biological reactions to occur (usually a number of hours), the mixed liquor is typically transferred to a separate gravity settling basin 18 (clarifier) to allow separation of the MLSS from the treated wastewater. Settled MLSS is then recycled to the aeration reactor basin as recycled sludge 16 to maintain a sufficiently concentrated microbial population for rapid degradation of the influent wastewater constituents. Because there is usually a significant net positive production of biological MLSS (the rate of cell synthesis exceeds the rate of cell destruction) an increasing inventory of sludge solids builds up in the system and the excess activated sludge 19 must be discarded or “wasted” from the process continuously or periodically. Wasting typically is from the secondary gravity clarifier or sludge recycle stream, but direct removal from the aeration reactor basin is also an alternative. The final result of the activated sludge process is two separate streams: the treated effluent wastewater 20 and the excess waste activated sludge stream 19. The effluent is a liquid/water stream very low in suspended solids content and soluble pollutants that is sometimes further treated to improve water quality prior to being discharged back into the environment. Further treatment options for the activated sludge treated wastewater effluent include nutrient (nitrogen and phosphorus) removal processes and sterilization through ozonation or by UV radiation.
The waste activated sludge stream from the secondary wastewater treatment process is very high (compared to the influent wastewater) in solids content—about 0.5-1.5 wt % total solid prior to thickening and about 2-3 wt % total solids after thickening. The waste activated sludge is often combined with sludge from a primary clarifier operating in front of the activated sludge process. It is highly desirable to process the total primary plus waste activated sludge in such a manner that it can be readily and economically disposed of without creating further pollution of the ecosphere. Further wastewater sludge treatment usually leads to either a concentrated liquid that can be land applied as a soil reconditioner, a stabilized solid biomass that is landfilled, or a pasteurized biosolid that can be beneficially used in some manner such as for a fertilizer or as a composting material.
The basic aim of all wastewater sludge treatment processes is to economically and efficiently reduce and stabilize waste sludge solids. In addition, the sludge treatment system should desirably also produce an end product which is fully suitable for final disposal without further physical or chemical treatment. In conventional practice final sludge disposal is commonly carried out by incineration, land filling or land spreading. In many instances, land disposal is employed and is particularly attractive due to minimal long-term environmental effects and is highly advantageous in contributing to reconditioning of the soil. However, the use of land spreading as a final sludge disposal method may require a well stabilized and pasteurized end product, so that the concentration of pathogenic organisms in the sludge is sufficiently low to avoid it becoming a health hazard. Also, the sludge should be adequately stabilized to prevent further degradation in the environment and the attraction of vectors.
Traditionally, three distinct processes have been widely utilized for treating wastewater sludges: oxidation ponds, anaerobic digestion and aerobic digestion. Oxidation ponds are generally employed in the form of comparatively shallow excavated earthen basins which extend over a large area of land and retain wastewater prior to its final disposal. Such ponds permit the biological oxidation of organic material by natural or artificially accelerated transfer of oxygen to the pond water from the ambient air. During the bio-oxidation process, the solids in the wastewater are biologically degraded to some extent and ultimately settle to the bottom of the pond, where they may become anaerobic and are further stabilized. Periodically the oxidation pond must be drained and the settled sludge dredged out to renew the volumetric capacity of the pond for further wastewater sludge treatment. The withdrawn sludge is utilized for example as landfill. Oxidation ponds thus represent a functionally simple system for wastewater sludge treatment. The use of oxidation ponds, however, has limited utility, since their operation requires sizable land areas. Moreover, no significant reduction of the level of pathogens in the sludge and only limited reduction in the quantity of the waste sludge is accomplished by this elementary treatment and disposal method.
Anaerobic digestion has generally been the most extensively used wastewater sludge digestion process for stabilizing concentrated organic solids, such as are removed from primary settling tanks, biological filters and activated sludge plants as discussed above. In common practice, the combined excess waste sludge is accumulated in large covered digesters where the sludge is mixed and naturally fermented anaerobically for about 30 days. The major reasons for the widespread commercial use of anaerobic sludge digestion are that this method: (1) is capable of stabilizing large volumes of dilute organic slurries, (2) results in significant biological solids (biomass) reduction and stabilization, (3) produces a relatively easily dewaterable final sludge for ultimate disposal, (4) is a net producer of methane gas, and (5) is potentially capable of producing a pasteurized sludge under the right conditions. Anaerobic digestion is characteristically carried out in large scale tanks which are more or less thoroughly mixed, either by mechanical means or by the recirculation of compressed digester gas. Such mixing rapidly increases the rate of the sludge stabilization reactions by creating a large zone of active decomposition.
Methane gas is produced during anaerobic digestion and is characteristically used in combustion heaters to offset heat losses of the anaerobic digestion process which usually operates at above ambient temperatures. However, seasonal temperature variations and fluctuations in the suspended solids level of the influent wastewater sludge have a significant effect on both the rate of methane gas production and the amount of heating which is necessary to maintain the digestion zone at the desired elevated temperature operating level. As a result, if elevated temperature conditions are to be maintained year round in the anaerobic digestion zone, an auxiliary heating system is generally an essential element of the overall sludge digestion system.
Since the rates of anaerobic digestion and resultant methane gas formation are strongly influenced by the suspended solids content of the sludge undergoing treatment and by the temperature level in the digestion zone, it is in general desirable to feed as concentrated a sludge as possible to the digester, thereby minimizing heat losses in the effluent stabilized sludge stream discharged from the anaerobic digester while maximizing methane production in the digester. However, even with such provisions elevated temperatures are difficult to maintain economically in the anaerobic digestion zone, especially during winter months. Furthermore, even comparatively small temperature fluctuations in the anaerobic digestion zone may result in disproportionately severe process upsets and souring of the digester contents, as is well known. Perhaps the most important disadvantage of anaerobic sludge digestion systems is the requirement for large residence times of about 30 days that are needed to achieve adequate stabilization. These large residence times result in very large tank needs and correspondingly large capital costs for tank construction and mixing.
As an alternative to the foregoing anaerobic methods, biodegradable wastewater sludge can be digested aerobically. Air and to a much lesser extent high purity oxygen has been employed in practice as the source of oxygen for this purpose. It is also well known that aerobic digestion proceeds more rapidly at elevated temperatures. As the temperature rises above 40° C., the population of mesophilic microorganisms declines and thermophilic forms increase. The temperature range of about 50-70° C. is often referred to as the thermophilic range where thermophilic bacteria predominate and where most mesophils are extinct. Above this range, the thermophils decline, and at 90° C., the system becomes essentially sterile. Because of the more rapid oxidation of sludge biomass at higher temperatures, thermophilic digestion achieves more complete removal of biodegradable volatile suspended solids (BVSS) than the same period of digestion at lower temperatures. A more stable residue is obtained which can be disposed of without nuisance. Thermophilic digestion can also effectively reduce or eliminate pathogenic bacteria in the sludge (pasteurize the sludge), thereby avoiding the potential health hazard associated with its ultimate disposal.
When air systems are used to supply dissolved oxygen for aerobic sludge digestion systems, with the air being passed through the body of sludge liquid in a digestion tank and freely vented to the atmosphere, the loss of heat from the sludge to the air being passed through the digester tends to become substantial in magnitude. This loss of heat is due in part to the sensible gas temperature heat loss of the hot gas being discharged into the atmosphere, but more importantly due to the evaporative heat loss of the gas caused by the evaporation of substantial quantities of water into the gas phase during the oxygen dissolution process. The air being contacted with the higher temperature sludge biomass will quickly come to the temperature of the sludge biomass being aerated and will also rapidly evaporate enough water into the gas phase to quickly bring the water content of the gas phase into gas-liquid equilibrium with the sludge liquid from the standpoint of the water vapor content of the aeration gas. As a result, aerobic digestion in the past has often involved digestion with only lower temperature mesophilic microorganisms. However, more recently air sludge digestion systems operating in the thermophilic temperature range have become more common by employing such techniques as covered and highly insulated tanks, external heat sources and heat exchange equipment to minimize both the gas phase and liquid phase heat losses from the aerobic digester contents. Air contains only 21% oxygen and only about 10-20% of the oxygen content thereof is dissolved and available to the bacteria in an air aerated aerobic sludge digestion system. Accordingly, a very large quantity of air must be used to supply the oxygen requirements of the process and the heat losses from the digester associated with venting the sensible heat of the “spent” air and the latent heat required to saturate the spent air with water vapor are substantial. As a result of these heat losses in conventional air aerobic sludge digestion systems, very large quantities of external heat and/or extensive heat transfer equipment must be employed to sustain the sludge temperatures at the elevated thermophilic levels.
Several strategies have been employed to avoid the need for the addition of external heat into thermophilic aerobic sludge digestion systems. These methods are generally classified as autothermal aerobic digestion systems or “ATADs”. The ATAD process is an aerobic digestion process that achieves thermophilic operating temperatures without external supplemental heat beyond that supplied by the aeration and mixing energy. Within the ATAD bioreactor, sufficient levels of dissolved oxygen, volatile solids, and mixing allow aerobic microorganisms to degrade organics to carbon dioxide, water, and nitrogen byproducts, during which significant heat energy is released and absorbed into the liquid phase. If sufficient insulation, residence time, and adequate solids concentrations are provided, the process can be operated at thermophilic temperatures to achieve a high level of volatile solids destruction and pathogen reduction sufficient to meet U.S. EPA regulations for the 40 CFR Part 503 Class A designation.
Since the early 1980s the U.S. EPA has promoted the use of biosolids in agriculture and issuance of the 40 CFR 503 regulations in 1993 further encouraged the practice. These regulations require that any biosolids applied to land must meet certain pathogen and vector attraction reduction limits. For example, the Class A designation specified in the regulation requires that pathogen levels have been reduced to below detectable levels. The regulations provide for six alternatives for meeting the pathogen reduction requirements. As an example, one alternative is to ensure that all particles are processed for a time determined by the following equation: D=50,070,000/100.14t which applies when total solids are <7%; t is ≧50° C.; and D is residence time which is ≧30 minutes. The second requirement of the regulations is related to stabilization or vector attraction reduction. The regulations give at least 10 options for meeting vector attraction reduction. One example is a 38% reduction in the volatile solids component of the sludge.
Air ATADs have been known for about two to three decades. FIG. 2 shows a schematic for a conventional type of Air ATAD system. Liquid feed sludge 22 is first thickened usually in a thickener 23 producing an at least about 3% by wt solids underflow stream 38 that enters one or more of the ATAD reactors 24. The thickener also produces an overflow liquid stream 39 having a much smaller percentage of solids and is typically recycled back to an aeration basin. The reactors are typically enclosed and insulated. They also include mixing, aeration means, 25 and foam suppression equipment and are operated in batch mode with a sludge retention time of from about 5-10 days. These ATADs typically operate with two tanks or bioreactors in series but are not operated in a continuous flow manner. Some stabilization and heating occur in the first tank, with further stabilization and heating to temperatures of about 55° C. to 65° C. occurring in the second tank. Feeding is intermittent, with removal of digested solids from the second tank, transfer of digesting solids from the first to the second tank, and addition of feed solids to the first tank. This promotes temperature elevation and minimizes short-circuiting of feed solids to the stabilized solids, thereby enhancing pathogen destruction. The digested sludge exits to a storage or cooling tank 26 before being further processed and/or land applied. The exit gas (offgas) 27 is vented or further treated such as by scrubbing. Benefits of ATAD include a high disinfection capability, relatively low space and tankage requirements, and a high sludge treatment rate. It is an effective and environmentally responsible means of achieving aerobic stabilization and producing sludge that meets the current regulations for Class A sludge pathogen control and for disposal of agricultural, municipal and industrial wastewater sludge on land and underground.
Single tank ATAD systems are also known that operate with a feeding technique called a partial fill and draw process where for example on a daily basis partial withdrawal from the reactor of about 1 days volume of sludge will occur for about 1 hour, then new feed will fill the tank back up followed by batch reacting for the remainder of the 24 hour cycle. This method limits the temperature swing of the system, but requires higher tank volumes. Digested sludge withdrawn from an ATAD can be further processed using conventional techniques such as dewatering prior to final disposal.
It is also known that heat losses in aerobic sludge digestion systems can be reduced by using oxygen-enriched or high purity oxygen gas rather than air. If a high utilization of the high purity oxygen gas can be achieved, the total amount of gas which must be fed to and vented from the aerobic digester is considerably smaller compared to air, because most if not all of the inert nitrogen gas has been removed. Heat losses due to sensible warm-up and to water evaporation into the high purity oxygen gas stream are also significantly decreased. These reductions in heat losses are sufficient for autothermal heat alone to sustain the temperature at levels appreciably higher than ambient, so that the digestion zone is able to operate efficiently in the thermophilic temperature regime with no input of external heat to the process. Since thermophilic stabilization is much more rapid than mesophilic stabilization, the necessary residence time to achieve adequate stabilization in the aerobic digestion zone is also greatly reduced in the thermophilic mode. This in turn permits the use of smaller reactors which further reduces heat losses to the surroundings. Because of the faster rate of oxidation of sludge, high purity oxygen ATAD can achieve suitably high biodegradable volatile solids reduction, in comparatively short sludge retention periods.
Despite their significant attractive features, ATAD systems have several associated disadvantages relative to anaerobic sludge digestion. First, since the thermophilic aerobic digestion process is oxidative in character, the process produces a bio-oxidation reaction product gas containing carbon dioxide and water vapor which have no end use utility and are directly vented to the atmosphere or scrubbed. By contrast, anaerobic digestion produces methane gas as a reaction by-product which may be exported from the treatment facility and is also useful as a fuel gas for satisfying the heating energy requirements associated with digestion at elevated temperatures. In addition, the aerobic digestion zone requires a much greater energy expenditure, for mixing and gas-sludge contacting, than is required in the anaerobic digestion system for mixing of the digester contents.
Many United States patents have been issued for improved aerobic sludge treatment processes operating in the thermophilic temperature range. Some representative examples include: U.S. Pat. No. 3,745,113 to Fuchs, U.S. Pat. No. 4,246,099 to Gould et al., U.S. Pat. No. 4,277,342 to Hayes et al., U.S. Pat. Nos. 4,975,194 and 4,983,298 to Fuchs et al., U.S. Pat. No. 5,587,081 to Norcross et al., U.S. Pat. No. 5,948,261 to Pressley, U.S. Pat. No. 6,068,047 to Buchhave, U.S. Pat. No. 6,203,701 to Pressley et al., and U.S. Pat. No. 6,325,935 to Hojsgaard. Several of these patented processes employ autothermal thermophilic aerobic digestion, or “ATAD”, technology to treat the sludge biomass.
While existing aerobic sludge digestion systems provide viable sludge utilization and disposal alternatives, they also have a number of limitations. Most notably, many systems are not a reliable and predictable means of producing a pasteurized (Class A) sludge that has beneficial environmental uses. Moreover, operational difficulties exist with some conventional ATADs, particularly because they are mechanically more complex, require larger tanks, require expensive heat transfer equipment, and/or are subject to severe foaming.
Over the years, many solutions have been proposed for improving the disposal of wastewater sludge and overcoming the limitations of ATAD. As evidenced by the variety of patents mentioned above, there continues to be a need for further improved designs. Thus, while much effort has been spent in development of improvements in sludge treatment technology as well as in refinement of existing sludge treatment processes, there still exists a great need for better and more efficient and effective sludge treatment and disposal systems. There is especially a need for an efficient, aerobic sludge treatment system that is capable of producing a Class A pasteurized sludge at lower operating and capital costs. These are the primary needs addressed by the present invention.
Accordingly, it is an object of the present invention to provide an improved process for aerobic thermophilic digestion of wastewater sludge.
It is also an object of the invention to provide an efficient multistage aerobic sludge digestion system that is operated in a simple to operate and reliable performance continuous flow manner.
It is also an object of the present invention to provide an aerobic sludge digestion system with improved efficiency and lower operating costs and/or lower capital costs compared to current systems.
It is a further object of the invention to provide an aerobic sludge digestion system that can destroy pathogenic organisms and organic matter within wastewater treatment sludge so as to reliably produce a Class A pasteurized and adequately stabilized sludge.
It is a further object of the invention to provide an aerobic sludge digestion system that can be operated autothermally in the thermophilic temperature range at significantly lower capital and energy costs without the need for external heat sources or heat exchangers.
It is another object of the invention to provide an efficient multistage thermophilic aerobic sludge digestion system that can be integrated in front of an existing anaerobic sludge digestion process.
It is another object of the invention to provide an aerobic thermophilic sludge digestion process employing aerobic digestion and anaerobic digestion at elevated temperature, in a manner which utilizes the advantages of each while minimizing their attendant disadvantages.
It is a further object of the invention to provide a multistage sludge digestion system with the ability to specifically select the gas to liquid contacting staging order to optimize the overall performance of the entire system.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.